Trailer brake control

ABSTRACT

An electronic braking system (WBS) suitable for commercial vehicles which tow trailers, which has on the towing vehicle, braking control of all the axles and a device for setting the braking demand for the trailer which is so adjusted as to equalize the utilization of adhesion between said towing vehicle and trailer. The system includes two adaptive control loops ( 30   a   , 30   b ) which are adjusted sequentially in a co-ordinated manner in order to set respectively the towing vehicle and trailer braking to levels where overall train mean decelerations approximate closely to those corresponding levels demanded by the driver and at the same time set the distribution of braking to seek to achieve equal utilization of adhesion between the tractor and trailer. The system also uses deceleration errors to sequentially adjust the two adaptive control loops.

The present invention is concerned with trailer brake control systems inmotor vehicles, which operate without the use of additional onboardsensors.

The requirement for a full control of a towing vehicle (tractor) and asemi-trailer during braking is such as to generate the need fordeceleration control of the tractor in combination with a simultaneousadjustment of trailer braking levels so as to achieve coupling forcecontrol in a manner such as to cause each of the tractor and trailer toexert a fair share of the overall braking effort. This situation isequivalent to an equal utilization of adhesion between tractor andtrailer and is recognized as being efficient on lining wear and ingenerating the most stable condition during braking, since skidding willonly result from reduced surface adhesion and not from unbalancedbraking distribution. Reduced lining wear results from the equitablebraking balance in energy absorption which keeps the lining temperaturesat their lowest mean level.

EP0303 827 teaches a dual mode trailer control system employingproportional braking as described above for best stability at higherdeceleration levels plus, at lower braking demands, a balanced brakingregime under which braking is made at levels which depend upon the sizeand number of axle brakes. However, this approach is not suited tounknown trailer types where numbers and sizes of brakes can varysubstantially as different trailers are picked up during roadoperations.

The requirement of proportional braking distribution between tractor andtrailer is the preferred mode in the industry and a number of methods ofachieving this have been suggested, in the main requiring measurementsof coupling force in two planes for semi-trailers.

Our EP0370 671 discloses a means for controlling the brakingdistribution and overall combined vehicle deceleration but requires someform of measurement of vehicle deceleration-and of coupling forces inorder to achieve these ends simultaneously.

EP0433 362 disclosed a trailer control system in which errors betweendemand and achieved deceleration automatically cause the trailer brakingproportion to be adjusted accordingly. This makes the assumption thatany errors in combined vehicle braking must have resulted fromvariations in trailer braking only. In practice, this is not always thecase as the towing vehicle brakes are subject to the same sources ofvariation in performance as the trailer brakes and correction when madeshould be directed to that part of the combination which contains thesource of braking error levels. Many cases exist where there arevariations in both parts of the combination, maybe to different extents,and the present invention attempts to focus suitable correctiveadjustment to both said parts of the vehicle in a co-ordinated manner soas to prevent wrongly applied corrections having to be later reversed asthe other vehicle half is adjusted. An important feature of the presentinvention is the requirement for measurement only of deceleration of thecombination, saving the significant cost of measuring the couplingforces or of deriving these forces from tenuous processing of vehicleaxle load changes and interpreting these from serious levels of noisegenerated by road Irregularities which are a part of practical vehicleoperation.

It is envisaged that trailer braking parameter variations are likely topredominate in sourcing vehicle retardation variations and therebycausing braking errors. However, considering the case of a towingvehicle with better than expected braking performance linked to atrailer with under-performing brakes leads, a simple control systemmight conclude from the low deceleration errors, that the combination iscorrectly braked when the distribution is quite clearly in error.

An object of the present application is to provide a system which,although employing only deceleration measurement, has the ability tocorrect braking in both parts of the vehicle and thereby control bothoverall deceleration and braking distribution.

The control of semi-trailer braking requires that the system be able toexamine the performance of each part of vehicle but this cannot beachieved totally independently. However, it is practical to examine thetractor or trailer performance alone if done with care and then toexamine the combined performance in order to assess the brakingeffectiveness of the other part of the vehicle, so as to be able toapply individual corrections to each part as described above.

Examination of the trailer-only performance can be undertaken preferablyafter combination braking is established because of the delay inresponse of the trailer brakes which would make it appear that thevehicle brakes were not responding. Thus all axle braking, onceestablished, would revert to trailer-only braking by slowly removingtractor braking while at the same time increasing trailer braking tocompensate for this loss in total braking effort which would otherwiseexist. Trailer braking effectiveness is then assessed by relatingoverall deceleration achieved to the driver demand, allowing for thetotal combination mass and the boost in trailer braking.

However, since more parameters are known on the tractor and are moreconstant than trailer characteristics which could change with every newtrailer picked up, it is deemed preferable to brake the tractor-only oncertain stops and then to assess the trailer performance when brakingthe combination, The present Application will concentrate on thisstrategy but recognises that the trailer-only alternative is technicallyequally possible.

The performance of the tractor-only is most easily assessed -byexamining the retardation against demand, for the tractor without atrailer coupled and this will be done whenever the opportunity presentsbut it is recognised that some vehicles never operate in this mode andfor those that do, the braking pressures ate so low, particularly on therear axle(s) due to the light loading that it does not represent a realtest of serious braking.

In accordance with the present invention there is provided an electronicbraking system (EBS) suitable for commercial vehicles which towtrailers, which has on the towing vehicle, braking control of all theaxles and a means of setting the braking demand for the trailer which isso adjusted as to equalize the braking effect at the road surfacebetween said towing vehicle and trailer, characterized by the employmentof two adaptive control loops which are adjusted in turn in a manner toachieve. single direction co-ordination between the loops whereintractor braking errors are allowed for in the braking of the trailer inorder to set respectively the towing vehicle and trailer braking tolevels where overall train mean decelerations approximate closely tothose corresponding levels demanded by the driver and at the same timeset the distribution of braking to seek to achieve equal braking effectat the road surface between the tractor and trailer, furthercharacterized by the use of deceleration errors in said repeatedadjustment of the two adaptive control loops.

In a preferred embodiment, the towing vehicle adaptive control loop isadjusted after selected vehicle stops on the basis of the measured errorbetween the drivers braking demand and the deceleration of thetractor/trailer combination when braked by the axles of the towingvehicle alone.

Advantageously during periods of towing vehicle-only brakingdeceleration is maintained with the normal proportion to the demand, byan increase of towing vehicle braking so as to provide additionalbraking forces to compensate for the absence of trailer braking, saidincrease being set in relation to the ratio of mass (Mt) of the rearpart of the trailer which is normally braked by the trailer brakes, tothe total mass (Mf+Mr) being supported on the towing vehicle axles.

Preferably, the towing vehicle-only braking of the combination takesplace only at predetermined but variable intervals and only on selectedbrake applications where the braking demand is within a preset low band,the vehicle is not subject to any substantial steering input and thespeed at the commencement of braking is above a preset threshold yetbelow an upper limit set by safety considerations.

It is also preferred that the trailer control loop is subject toadaptive adjustment on selected stops other than those of the lastparagraph, based upon measurements, made during a selected part of eachsuch stop, of the error signal between deceleration demand and theactual achieved deceleration of the combination.

In a still further preferred arrangement, errors detected during thetowing vehicle-only braking phases are not only used to adapt the towingvehicle adaptive loop but are also stored and a percentage of thiscurrent figure is used to adjust the braking demand input to the trailercontrol loop in order to co-ordinate the adaptive adjustment of thislatter loop to the adjustment of the towing vehicle loop by compensatingfor errors in the towing vehicle loop.

The total mass of the tractor/trailer combination can be assessed fromthe engine output torque and the gear ratio operating between the engineoutput shaft and the rear wheels as based on the relative speeds ofthese shafts and the rear wheel radii, during acceleration of thevehicle, this assessment taking place over several suitable accelerationphases with a running average being continuously updated as the vehiclejourney continues.

In the event of any ABS operation, it is preferred that any towingvehicle-only braking is immediately inhibited and furthermore anymeasurement, which is not complete is abandoned whether this relates tothe towing vehicle alone or to the whole vehicle from which the trailerperformance would be derived and the adjustment of any adaptive constantis prevented.

Preferably, the towing vehicle and trailer have separate adaptive loopswhich are sequentially adjusted, in which each loop has two parameters(constants) which are representative of braking threshold pressure andthe slope of the transfer function characteristic relating decelerationto braking pressures for the appropriate vehicle half.

Upon detection of conditions which indicate the removal and change ofthe trailer, the adaptive parameters of the trailer system arepreferably reset back to pre-programmed starting values while at thisevent the corresponding towing vehicle parameters are not changed andthe learning process continues without a break as running is commencedwith the new trailer.

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 shows the sectionalized masses of a typical semi-trailercombination vehicle which has been considered in the systemorganization;

FIG. 2 shows a typical relationship between demand and axle pressure fora typical set of brakes which is needed to achieve that decelerationlevel;

FIG. 3a outlines the dual adaptive loops with the co-ordinatingfeedforward therebetween in a system in accordance with this invention;

FIG. 3b illustrates the components of each adaptive loop able to adjustto the pressure threshold of the axle brakes and adapt the slope of thepressure characteristic;

FIG. 4 shows a flow-chart for generating the tractor-only brakingdecision; and

FIG. 5 shows a typical commercial vehicle electronic braking systemwhich would incorporate the dual adaptive loops in the main ECU.

As discussed hereinabove, the main strategy selected in theimplementation of the present invention is to occasionally brake thewhole combination on the tractor brakes alone and for this operationthese brakes are boosted in order to generate additional braking forcesto retard the trailer at the demanded deceleration level. This isreferred to hereinafter as the (trailer) mass corrected setting for thetractor only. Such stops are only occasional and only allowed to takeplace under selected conditions in order that safety during the brakingoperation will not be prejudiced. Normal braking makes use of all brakeson the whole tractor/trailer combination and it is on these stops thatthe braking performance of the trailer is examined and it is animportant feature of this scheme that the adaptive correction to trailerbraking levels is made with reference to the errors recorded in earliertractor-only braking operations. This means that if the tractor-onlybraking operation has shown a deceleration error, then a large part ofthis error level is injected into the trailer measurement in order toequalize the error in both systems as a means of giving priority tobraking balance between tractor and trailer and then correctingdeceleration gradually over a series of subsequent stops.

From a braking aspect, a typical tractor/trailer combination in shown inFIG. 1 and can be separated into three mass sections as follows:

Mf supported & braked by the tractor front axle.

Mr supported & braked by the rear tractor axle/bogie.

Mt supported & braked by the trailer rear bogie

The mass section Mr, while including the tractor rear elements is madeup mainly of the trailer front section and the load carried thereon. Itis this element which exerts a downforce Fv which is significant in theladen case, and which during braking must be retarded by countering thethrust Fh. The measurement of this downforce is made by sensing the loadon the rear axle(s) of the tractor and subtracting the axle tare weightof the tractor rear section which remains sensibly constant and beingknown from the build stage is pre-set into the system, The front mass issimilarly constant and again is pre-set into the system as a post-buildparameter.

In some tractors, particularly those with a single rear axle, the pointof application of force Fv is in front of the rear axle and thus acomponent is applied to the front axle acting effectively as an increasein mass Mf. The fraction of this Fv force which acts in this way isdetermined by dimensions and is set into the system by post-buildprogramming, ie KFv acts on the rear axle and is sensed and (1−K) Fvappears on the front axle and can be inferred from the increase in rearaxle load. The trailer mass Mt is unknown because it depends on theplacing of the trailer load and is unmeasured on a standard trailer.However, it can be derived from the total mass of the combination whichis assessed during acceleration since it is impractical to attempt tomeasure it during braking when the trailer braking contribution isunknown. During acceleration, the engine management system suppliesoutput torque or power signals and these are coupled to the operatinggear ratio measured from engine and wheel speeds taking into account therolling radius of the drive wheels.

When braking the whole vehicle on tractor brakes only, these are boostedin order to restore pedal response in the absence of trailercontribution. This boost or trailer mass compensation is calculate onthe basis of mass elements as follows:

Front axle pressure Pf is normally set as:Pf = Pfb + Pc⁺d[(1 − K)Fv + Wf]Rear axle pressurePr   is  similarly : −Pr  = Prb + Pc⁺d[Kfv + Wr]where: − Pfb&Prb  are  P  thresholds.d = demand  deceln.Wf = Front  tare  load.Wr = Rear  tare  load.Pc = pressure  const.  in  bar/tonne/gKv = coupling  geometry  factor.

These levels are increased to:

Pf′=Pfb+Pc*d[(1−K)Fv+Wf](1+Rm)

where:

Pr′=Prb+Pc*d[KFv+Wr](1+Rm)

${1 + {Rm}} = {\frac{\text{Total~~mass}}{{Mf} + {Mr}}\quad {see}{\quad \quad}{{FIG}.\quad 1}}$

The front and rear threshold pressure Pfb & Prb are preset in ratio froma knowledge of the braking equipment fitted to the tractor and areadapted together as a single quantity as will be explained later.

Because tractor-only braking can be dangerous in some circumstances, itwould never be applied in vehicles which were not fitted with ABS oreven used on vehicles in which the ABS was not functioning correctly. Inaddition, tractor-only braking takes place only on selected brakeapplications at predetermined application intervals which are set at anapproximate frequency but this is increased where significant error intractor braking performance is exposed. This frequency is again reducedonce the adaptive process has reduced the error to a low level.

The final decision, upon a brake application being commanded, eventhough a tractor-only application is scheduled, is set to depend uponthe following safety override conditions:

a) The mass compensated demand represents a deceleration demand which islower than a predetermined limit typically lying in the range 0.2-0.3 gyet greater than a lower limit of 0.15 g or thereabouts, below whichmeasurement is unreliable.

b) There is no significant steering input as detected from the steeringwheel angle or differential wheel speeds.

c) The vehicle speed at the commencement of braking lies in a presetband typically 30-70 km/h.

When a brake application is selected, the mass compensation increase isapplied and the tractor-braking mode is entered if the conditions aboveare all met. If not, a normal application is taken and the tractor-onlymode deferred until the next application. When the tractor-only mode isentered, the brakes are applied only for a preset time period limited totypically 2 sec after the demand has . stabilized. After this point intime, the mass compensation is gradually reduced and the trailer brakedemand is gradually increased so that full balanced braking continuesfor the rest of the stop. However, the only assessment which is made onthis stop is that of tractor-only braking in which the decelerationerror is examined in the 2 sec period.

If the demand changes significantly during this period or there is anydetection of skidding, the period is ended immediately and full vehiclebraking takes over and the tractor-only phase will be repeated on thenext suitable stop.

During this 2 sec assessment period, errors between the braking demandand the level of achieved deceleration are accumulated once the demandhas stabilized. The mean error figure is stored and used to update theadaptive constants which relate to the tractor axle braking pressures tothe demand input. These ‘constants’ are used for each section of thevehicle and are specifically a pressure threshold correction and apressure constant modifier which starts out at a nominal figure and isincreased for poor braking or reduced for better than expected braking.

The effect of this adjustment mechanism on the adaptive loop is shown inFIG. 2 which is a plot of axle pressure against deceleration demand. Thepressure characteristic shows that for both tractor and trailer,pressure must be subject to an initial step as soon as the minimumdemand is exceeded and thereafter increases linearly with demand. FIG. 2shows that at low demands/pressures the errors detected are used toprovide data which adjusts the threshold point, while at higher demands,errors give data which adjusts the slope of the pressure/demand transferfunction: where:

P=a*d+C.

a=Pressure Const.

C=threshold step

Thus the threshold step and Pressure constant terms will be adapted fromerror signals and the system is programmed to accent corrections at apreset fraction of the correction levels generated on any particularstop. In this way corrections cause the characteristic to be graduallyadapted and if the chosen fraction is made proportional to thecorrection amplitude the adaptive or learning process becomesexponential and over-correction followed by re-correction will bereduced.

Co-ordination of the adaptive adjustment of the towing vehicle andtrailer braking systems is a preferred feature of the present inventionin order that the two loops can be sequentially adapted without anyexcessive interaction thereby achieving the quickest and most stablesettling regimes. This co-ordination is not the mutual co-ordination ofEP B 0370 671but a single direction co-ordination wherein the tractorsystem errors are allowed for in the trailer system.

Taking the case where the tractor is overbraked, tractor-only brakingwill expose an excess deceleration which, given a perfect trailer, wouldlead to some over braking of the tractor/trailer combination andconsequent reduction of trailer braking levels. “Co-ordination” in termsof this present invention is the means whereby the excess decelerationof the tractor braking alone is stored and a percentage of this figureproduces an adjustment to the effective braking demand used in the errorassessment of the trailer system The actual percentage used depends onthe mass ratio Rm.

Rm is given by: Mt/(Mf+Mr).

1/(1+Rm)=(Mf+Mr)/whole mass.

This excess deceleration is multiplied by this factor

1/(1+Rm)

and generates the reduced excess deceleration expected from a perfecttrailer when braked along with this tractor. This is the component whichis added to the demand for comparison with the achieved combinationdeceleration to determine the trailer error. As the tractor is adaptedback to the correct braking level, this trailer offset iscorrespondingly reduced to arrive at an undistorted error signal anddeceleration errors are gradually reduced to zero for both vehiclehalves.

Referring now to FIG. 5 there is illustrated by way of example theElectronic Braking System organization on a tractor unit capable oftowing a standard trailer and adjusting the trailer braking pressures bymeans of an electronically controlled trailer valve 8 on the towingvehicle or communicating to an EBS trailer via a special data link CANt.The EBS on the tractor unit includes brake actuators 6. In thisparticular system, used as an example, a 3-channel EBS is employed togive axle control on the tractor steering axle and wheel control on thedrive axle, primarily to improve the split mu ABS performance.

Braking demands are transmitted to the main ECU 10 from a transducer 12incorporated in the pedal valve assembly. The adaptive section of thepresent system is built in software terms within the computer core ofthe main ECU 10 which communicates with smaller ECUs 14,16,18 associatedwith each of the pressure control valves via a high speed data exchangesystem 20. The smaller ECUs have the task of handling the peripheralsignals and performing the local control of pressure for the brakes,from demand signal issued from the main ECU.

Wheel speed signals from wheel speed sensors 22 are converted withinthese ECUs and transmitted back to the main ECU 10 for monitoring andhigher level purposes. The main ECU 10 also receives the vehicledeceleration signal from an on-board decelerometer 24 which, in certainlower cost applications would be replaced by a differentiated vehiclereference speed signal. Unfortunately, the result of this replacementincurs the loss of automatic gradient correction which would result inerror in some measurements made on hills. However, this is notsufficient to invalidate the control principles since in a graduallyadapting system, gradient errors would tend to cancel over any period ofnormal road operation at the expense only of increased settling time.The rear axle load is sensed by a load sensor 26 and passed to the mainECU 10 where it is used to generate braking proportional to load beingcarried and to give a measure of the variable component of mass Mr. Mfis largely obtained from stored build data but in some vehiclesincorporates also a component of the trailer load as determined from thecoupling position in relation to the rear axle.

Trailer braking is controlled from the main ECU 10 and can utilizeeither electronic or pneumatoic signaling or both. However, to cater toperfectly standard trailers known today, an electronically controlledtrailer valve 8 as shown in FIG. 5 is needed. Given either means ofsignaling, the level of trailer braking is set from the main ECU 10which takes into account the driver demand from the pedal transducer 12as a master control signal but modifies this by taking into account thecurrent state of the adaptive “constants”, one of which may be subjectto some adjustment after completion of the brake application.

The adaptive system is shown in FIGS. 3a and 3 b for a dual parameteradaptive correction organization. The separate loops 30 a and 30 b ofFIG. 3a are for tractor and trailer and both have deceleration error asthe input signal. However, assessment is made individually andsequentially depending upon the state of the tractor-only braking signalinput. For normal stops enable and inhibit switches 32 a.,32 b are inthe position shown in FIG. 3a so that the trailer adapt loop 30 b willbe affected by the deceleration error occurring in the selected part ofthe stop. Occasionally circumstances require the selection of atractor-only stop in which the early part of the stop is made by brakingthe tractor alone and during this period the input signal is activatedto disable the trailer loop 30 b and enable the tractor loop 30 a whileat the same time boosting the braking demand level as describedhereinafter in the trailer mass compensation feature. During a shorttime period once the demand has stabilized the tractor adaptive loop 30a assesses and stores the average error between demand and achieveddeceleration levels. This stored error is used to modify the appropriatetractor loop ‘constant’ after the end of the stop.

It should be noted that after the assessment period of typically 2 sec.this loop 30 a is disabled and the trailer mass compensation is reducedto zero at a controlled rate while the trailer braking is built up at asimilar controlled rate, although slightly in advance of the boostremoval, so as to compensate for delays in the trailer system.

As shown in FIG. 3a, the tractor loop corrective output signals forthreshold C and slope a, are routed to the front and rear axle pressurecontrol loops. The co-ordinating output formed from the stored errorfigure amended by the factor:

Mf+Mr)/(Mf+Mr+Mt), is provided as a third input to the decelerationerror calculation for the trailer adaptive loop 32 b. This provides themechanism whereby the trailer loop 30 breceives a compensatingadjustment if a tractor braking error exists. In the event of atractor-only braking phase, the tractor loop 30 a is the only one toreceive any subsequent adjustment even though the full braking durationmay be much longer than the 2 sec assessment period for the tractorloop. This is because it is expected that the combined vehicle may be insome deceleration error after the transition back to normal braking.

The action of the adaptive loops 30 a, 30 b is different in the event ofa change of trailers. Operation of the tractor without a trailer isrecognized by detection of very light rear axle loading (down to afigure near to the maximum tare weight). Such a condition is typical ofstatic conditions during a trailer change and the signal is used toreset the trailer adaptive adjustments to the nominal level programmedinto the system as post-build parameters while the tractor adaptive‘constants’ are allowed to remain at the current stored level so as toleave this learning process uninterrupted.

The internal organization of the adaptive loops 30 a, 30 b is expandedin FIG. 3b and shows the receipt of deceleration error as the main inputsignal but includes subsidiary timing and mean deceleration levelsignals. Error adjustment is made by adapting one of two variables C ora, as shown in FIG. 2, based on the mean braking demand registered overthe assessment period. At low level demands, the threshold level C isadjusted while at higher levels of demand the slope of thecharacteristic a, is adjusted. At all times, the current levels of thesetwo ‘constants’ are used to set the pressure demands to tractor andtrailer brakes in association with driver demand and, where appropriateaxle load. The error level on any stop is accumulated and averaged asset by the timing signals which select a period after the settlement ofthe demand, which is short for the tractor-only braking phase and can belonger for normal full vehicle stops provided that there are no seriouschanges to demand or no ABS operation is produced. The error average isprocessed after the brakes are released, to assess the pressurecorrection needed to reduce the error, as based on the threshold C andslope a values being used in the transfer function characteristic shownin FIG. 2. The amplitude of the pressure correction calculated whichwill be used to adjust the appropriate parameter C or a, is a variableand preset into the system to adjust the adaptive response so as togenerate a learning process which achieves a stable adjustment overseveral stops.

FIG. 4 is a simplified flow chart which explains the selection process,within the main ECU of the tractor-only braking phase, making testsnecessary for initiation of the phase and then timing the shortassessment period. After this period, the tractor and trailer brakinglevels are merged in that both systems receive gradually adjusteddemands over a following period of typically 1 sec, so as to achieve anormal distribution of the combined vehicle braking designed to betransparent to the driver. Thus the tractor reduces braking and thetrailer builds up braking from zero as shown in the inset waveformdiagram of FIG. 4

The flow-chart shows a section of program which is active only when adriver demand for braking is detected at which point the stop count istested against the value N which marks the point of the tractor-onlybraking phase. A count of N or greater means that a tractor-only brakingphase is due but it should be noted that N is not constant but has acomponent which is inversely proportional to the level of tractor errorwhich was stored on the last stop. In this way the frequency oftractor-only stops is reduced as the braking error falls. Where theschedule is such as to call for a tractor-only stop, the vehicle speedis checked to see if it lies in the 30-70 km/h band where braking of onehalf of the total vehicle is both useful and safe. If this is the case,a test of the steering input is made and if zero or near zero,tractor-only braking is sanctioned or else normal combination braking issubstituted. If tractor-only braking is given the go-ahead, the trailermass compensated demand is calculated and tested to lie typically in therange 0.15-0.3 g in which conditions for assessment are deemed suitable,then the tractor-only braking flag is set, the tractor adaptive loop isselected and the duration timer is started once the demand becomesstable. This timer is incremented in a timing/clocking sub-routineexternal to the program shown but well known in real time systems. Theskid detection subroutine is accessed and a test is made on all wheelspeeds to check for incipient skidding conditions and, if found at anywheel, the tractor-only braking is abandoned in favour of the completevehicle. If no skidding is detected, the tractor-only brakes are Drivenwith the increased demand which continues for the 2 sec period of theduration timer or until the demand falls to zero. An additional test ismade during this period and during the following merge period to detecta sudden increase in driver braking demand. If a demand in excess of0.45 g is found, then tractor-only braking or the following mergeprocess is immediately switched to braking of the full vehicle in orderto respond to what might be an emergency. Duration and stop counters arereset where appropriate.

Thus tractor-only braking continues and error data is collected,normally for 2 sec, after which the program switches to merge thetractor and trailer braking levels over many cycles through theseprogrammed steps, gradually increasing trailer braking and, with a shortlag, reducing tractor braking until the full vehicle braking levels areattained. This takes over approximately 1 sec after which the StopCounter is reset and the Duration Counter is cleared. If thetractor-only braking period is curtailed the stop counter is not reset,thereby setting up conditions for a next stop retry of this operationfor tractor assessment. Demand falling to zero is tested at each passthrough the program so as to generate a brakes release without delay.

The various steps in the flow-chart of FIG. 4 are summarizedhereinafter:

100 - Test braking demand

102 - Zero?

104 - Test stop count

106 ->N?

108 - Test vehicle speed

110 ->30&<70?

110 - Increment stop count

112 - Test steering input

114 - Small or 0?

116 - Calculate trailer mass compensated demand

118 - In range>0.15+<0.3 g ?

120 - Set tractor-only braking flag output to ADAPT LOOP SELECTOR startduration timer when demand stable

122 - Check for ABS activity

124 - Skid?

126 - Test duration timer

128 - ->2s?

130 - Drive tractor brakes only. Accumulate error data then test demandlevel.

132 - Zero?

134 - Reset duration timer

136 - >0.45 g?

138 - Increase trailer demand

140 - Reset stop count and duration timer

142 - Release brakes

144 - Reset duration timer

146 - Drive all brakes using normal demand

148 - Test demand level

150 - Zero?

152 - Reset stop count and duration timer

154 - Done?

156 - Merge tractor and trailer braking levels

158 - Test driver demand

160 - Zero?

162 - >0.45 g?

164 - Merge complete

166 - Reset stop count

The system described, measures the braking performance of each half ofthe vehicle in a sequence which is made sufficiently flexible topreserve safety when only one part is braked, yet designed to allow theadaptive process, particularly on linking up a different trailer, totake place quickly and effectively.

The change of a semi-trailer is detected by the fall in load sensed atthe tractor rear axle to a tractor-only level which is stored in thesystem memory as an initial static setting plus a margin for toleranceand additional equipment which might be installed. Alternatively,detection of a selected electrical circuit on the trailer is used togive an indication of trailer uncoupling if the load measurement couldbe ambiguous with other types of trailer.

In order to simplify the text, the aforegoing description has beenwritten to describe tractor and semi-trailer installation. However, theprinciple is extendable to full drawbar and centre-axle trailers withonly an alternative means of uncoupled trailer detection being provided.However, the division of mass Mf, Mr and Mt are slightly altered in thelatter case.

In a full trailer, the mass Mr is restricted to that part of the towingvehicle load which is supported by the rear axles(s) and should causeproportional braking on the towing vehicle, while the trailer mass Mtcomprises all that which is supported on the trailer front and rearaxles, with the distribution of braking between these axles being afunction of the trailer.

In a centre axle trailer, a small part of the mass of the trailer, up to1 ton, may act on the towing vehicle and must be sensed there, eventhough it is impossible to distinguish this component from the towingvehicle load. However the total mass of the combination minus the towingvehicle mass will still give the trailer mass which should be braked bythe trailer axles.

What is claimed is:
 1. An electronic braking system for a motor vehiclehaving a tractor towing a trailer, comprising: a brake inputcorresponding to a requested deceleration; a load sensor measuring agenerally vertical load on said tractor from a portion of said trailer;a deceleration device determining an actual deceleration of said motorvehicle; a processor having an adaptive routine comparing said actualdeceleration to said requested deceleration and calculating first andsecond adaptive control loops for setting tractor and trailer brakinglevels, respectively, said tractor and trailer braking levels beingpartially determined by said mass of said portion of said trailer; andfirst and second errors generated by differences between said actual andsaid requested decelerations with said first and said second errorsadjusting said first and second adaptive control loops, respectively. 2.The system according to claim 1, wherein said load sensor measures saidmass of a front portion of said trailer at a rear tractor axle.
 3. Thesystem according to claim 1, wherein said processor calculates a totalmass of said motor vehicle using an engine torque and a gear ratioduring an acceleration in combination with said generally vertical load.4. The system according to claim 1, wherein said deceleration devicedetects the change in speed of said motor vehicle over a period of time.5. The system according to claim 1, wherein said deceleration device isan accelerometer.
 6. The system according to claim 1, wherein saidprocessor includes at least one controller.
 7. The system according toclaim 1, wherein said processor includes software.
 8. The systemaccording to claim 1, wherein said first adaptive control loop includeinitial values for front and rear tractor brake pressures increased byan amount relating to a total mass of said motor vehicle.
 9. The systemaccording to claim 8, wherein said first error includes said differencesbetween said actual and said requested decelerations during tractorbraking only.
 10. The system according to claim 1, wherein said adaptiveroutine is executed during only during desirable vehicle operatingparameters.
 11. The system according to claim 10, wherein said desirablevehicle operation parameter includes said requested deceleration beingin a range of approximately 0.15-0.30 g.
 12. The system according toclaim 10, wherein said desirable vehicle operation parameter includes aninsignificant amount of steering input.
 13. The system according toclaim 10, wherein said desirable vehicle operation parameter includes avehicle speed in a range of approximately 30-70 km/h.
 14. The systemaccording to claim 10, wherein said desirable vehicle operationparameter includes a non-ABS braking condition.
 15. The system accordingto claim 1, wherein a percentage of said first error is utilized toadjust said second adaptive control loop and determine said seconderror.
 16. The system according to claim 1, wherein each of said errorsincludes a brake pressure threshold component and a brake sure constantcomponent.
 17. The system according to claim 1, wherein said first andsecond adaptive control loops are adjusted until said first and seconderrors converge to zero.
 18. The system according to claim 1, whereinparameters of said first adaptive control loop are maintained andparameters of said second adaptive control loop are reset in the eventof a trailer change.
 19. A method of distributing a braking forcebetween a tractor and a trailer of a motor vehicle, comprising the stepsof: a) requesting a deceleration; b) calculating a first adaptivecontrol loop corresponding to a tractor braking level; c) braking themotor vehicle with the tractor only; d) comparing the requesteddeceleration to an actual deceleration; e) generating a first errorcorresponding to a tractor braking error defined by differences betweenthe requested and actual decelerations; and f) calculating a secondadaptive control loop factoring said tractor braking error correspondingto a trailer braking level.
 20. The method according to claim 19,wherein step b) includes calculating a total mass of the motor vehicleusing engine torque and a gear ratio during an acceleration.
 21. Themethod according to claim 20, wherein step b) includes measuring agenerally vertical load on the tractor from a portion of the trailer.22. The method according to claim 21, wherein step b) includes measuringa front portion of the trailer at a rear tractor axle.
 23. The methodaccording to claim 19, wherein step d) includes measuring a motorvehicle deceleration with an accelerometer.
 24. The method according toclaim 19, wherein step d) includes calculating a change in speed of themotor vehicle over a period of time.
 25. The method according to claim19, wherein the first adaptive control loop includes trailer brakepressure threshold and pressure constant components.
 26. The methodaccording to claim 19, wherein second adaptive control loop includestractor brake pressure threshold and pressure constant components. 27.The method according to claim 19, wherein step f) includes modifying thesecond adaptive control loop by an amount corresponding to a percentageof a mass ratio of the motor vehicle.
 28. The method according to claim19, further including step g reducing the tractor braking level andincreasing the trailer braking level while the first and second adaptivecontrol loops are adjusted and the first and second errors converge tozero.
 29. The method according to claim 19, wherein step c) occurs onlyduring desirable vehicle operating conditions.